Uncaught NoMethodError: undefined method `Native' for main

[keshav.b...@gmail.com]

app.rb file ---

require 'native'

require 'opal'

$window = Native('window')

window.alert("hello world")

---

this ruby scripts gives the following error in jsconsole 

 Uncaught NoMethodError: undefined method `Native' for main

also I have included these in my html head tag

<script src="http://cdn.opalrb.org/opal/current/opal.min.js"></script>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.3/jquery.min.js"></script>

<script src="app.js"></script>

[keshav.b...@gmail.com]

Ohh sorry on the second line i forgot to put the $ sign, but in my script it exists. I still get the error